When the founder of a company returns after having stepped aside, he usually is aiming to save from ruin the firm he built. That was the case with Steve Jobs, Charles Schwab and Michael Dell.

But former New York City Mayor Michael Bloomberg finds himself in a very different situation as he comes back to run Bloomberg LP. The media company is hardly in crisis. It has solidified its position as a dominant provider of financial information, and its revenue and subscribers have jumped sharply in recent years.

There are threats looming, however. Mr. Bloomberg is returning to a more competitive marketplace than the one he left in 2002 and to increasingly strained relations with the financial institutions that make up the company's core customer base.

Bloomberg LP Chief Executive Daniel Doctoroff, who will step aside at the end of the year to make way for the company's founder, says he can't imagine the 72-year-old Mr. Bloomberg would have any trouble adjusting.

"Change is the only constant in this industry. Mike led this company through its first 20 years and while we may be bigger and more complex now, he won't find it any different because the strategy remains the same," he said.

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Mr. Bloomberg's reputation as a politically-savvy operator and shrewd businessman may help the company smooth over relationships with some of the biggest banks.

Tensions surfaced over the past year over a Bloomberg instant-messaging service that many traders use to communicate with one another; Bloomberg editorial staffers were able to access certain subscriber data. Bloomberg later disabled the newsroom's access to that information.

"Personally I look up to the mayor. I think he did a great job for the city," said Lloyd Blankfein, chairman and CEO of
Goldman Sachs Group.
"I think he will be even better coming back. He doesn't lead like someone his age, but like someone who is 10 years younger."

A day after announcing that he would take back control of the company he created 33 years ago, Mr. Bloomberg said there were no plans for big changes.

"Unsurprisingly, the most frequent question I've gotten this morning is 'how are things going to change?' The simple, honest answer is that they mostly won't. Our core strategy remains the same. We'll continue working hard to grow our market share," Mr. Bloomberg wrote in a memo to staff on Thursday.

A founder's return trip doesn't always succeed. Mr. Dell, for example, was unable to stem a decline at Dell Inc., the computer maker that he founded, despite his return to the CEO role in 2007. He later took Dell private.

Founders rarely resume command of a company that flourished during their absence, several leadership specialists said. The practice occasionally occurs because founders "miss the smell of battle," said Jeffrey Sonnenfeld, a senior associate dean at Yale School of Management.

Bloomberg LP now has 321,000 subscribers for its $20,000-a-year terminals, which supply a range of financial information, and is expected to generate $9 billion in revenue this year, making it the largest such data company in the world.

But while Bloomberg's annual growth has averaged nearly 6% over the past five years and it now controls 32% of the financial-data market, the company faces several challenges. Technological advances have made it easier for smaller firms like FactSet and Markit to compete against the financial-data giants, Bloomberg and
Thomson Reuters Corp.

Also, some U.S. banks have cut back in recent years on the number of data-terminal subscriptions. Bloomberg has targeted emerging markets to sustain its growth. The company is also expanding beyond terminals into data feeds and back-office operations that make up the plumbing of the banking system.

"When [Mr. Bloomberg] left, 90% of their revenue came from their terminal business, but with terminal subscriptions shrinking they have had to grow their business in other directions," said Doug Taylor, an analyst at Burton-Taylor International Consulting.

Big banks have grown more concerned in recent years that Bloomberg LP is encroaching on their turf by gathering vast amounts of data about the trading and pricing of securities.

Some Wall Street firms also are exploring alternative instant-messaging services, so they don't have to discuss sensitive information on Bloomberg's platform.

One recent entrant into the race to usurp Bloomberg's dominance in business chat is Wickr, a San Francisco-based startup that touts data privacy as one of its service's key features. Last month, The Wall Street Journal reported that a group of firms led by Goldman Sachs was in talks to buy an instant-messaging software developer, Perzo Inc., as a way to find an alternative to Bloomberg LP's service.

Mr. Blankfein said the move wasn't intended as a threat, nor did he think it would impact the bank's business dealings with Bloomberg LP.

"We're always doing things to create platforms and systems that make our lives easier. What we are doing is certainly not going to put a dent into anything Bloomberg does for us," he said.

Mr. Doctoroff said that the kind of tensions that have emerged in the past year are natural and have been a part of the company's relationship with its customers since the beginning.

"Relationships ebb and flow," he said. "That is just a constant part of the environment and we deal with it."